1. Field of the Invention
The present invention relates to a method and an apparatus for individually modulating a tube current of an X-ray computerized tomography (CT) system in CT examination of the heart that uses multi-row detector.
2. Description of the Related Art
In general, cardiac CT for coronary artery evaluation is used to observe calcification of coronary arteries by carrying out CT scan before administration of contrast medium (hereinafter, referred to as “pre-contrast”), and then, to determine a precise scan range (Z axis) and positions (X and Y axes). The imaging to observe this calcification is called calcium scoring CT imaging. It is typical to use a prospective ECG gating (electrocardiogram: hereinafter, referred to as ECG) technique and a Half-reconstruction technique (a technique for reducing a radiation dose and enabling image construction with only information obtained during about 180-degree rotation).
After the pre-contrast imaging, CT scan is carried out after the administration of the contrast medium (hereinafter, referred to as “post-contrast. In post-contrast imaging (image observed after blood vessels are enhanced by a contrast medium), typically a retrospective or prospective ECG gating technique (a technique for obtaining images with ECG signals that are outputted from heart and selecting heart phases with comparatively small motion) is used. The two studies (pre-contrast and post-contrast) show a significant difference in radiation dose. In such coronary CT angiography, coronary arteries, which are the targets for the CT angiography, are located on a surface of a heart, and move at a high speed (60 to 70 times per minute) by repeated contraction and relaxation of the heart. The size of the coronary arteries that need to be visualized is very small, and thus a technique for imaging them is complicated and cumbersome, and strongly depends on a variety of scan methods. With recent development of multi-row detector CT, indications as well as diagnostic precision have been improved, the frequency of such angiography has increased, and consequently, harmfulness relevant to radiation defined as a carcinogen has further emerged.
Recently, in addition to a technique of a CT system including a multi-row detector, there has been improved precision for finding significant coronary stenosis by means of coronary CT angiography with development of a technique for imaging a moving heart (significant coronary stenosis used here denotes stenosis of 50% or more).
Such an advantageous effect has been a great interest of researchers or the like in research for quantification of coronary stenosis and evaluation of usefulness of coronary CT angiography for analyzing constituent components of soft plaque observed in a patent who is prone to suffer from an acute heat attack (soft plaque: a substance generated on a blood vessel wall and mainly formed of a lipid core of which collagen-rich fibrous plaque or fat is a main substance, unlike calcified hard plaque).
In general, in an X-ray CT system that includes a multi-row detector and carries out helical scan imaging, an operator sets an imaging condition suitable for a patient before imaging, and the imaging condition is sometimes varied depending on objects to be imaged or purposes for which imaging is carried out. Namely, a variety of parameters such as tube voltage, tube current, X-ray beam thickness, size of FOV (field of view), image reconstruction slice thickness, helical pitch (speed of table movement), reconstruction algorithm, patient position, beam hardening, and method of ECG gaiting are set in accordance with the imaging condition, and then, a radiation dose for the patient is displayed under the selected imaging condition, thereby making it possible for a person who reads an image to judge adaptability of the imaging condition.
In general, in the X-ray CT system including a multi-row detector, a predetermined dose has been used in accordance with a body part to be examined, or the dose is determined by a measurement obtained from the patient such as weight, height or body dimensions, or the dose can be determined by a scout view (scout image: an image similar to that obtained by general X-ray imaging that is not tomography imaging, and acquired without rotation of X-ray tubes). However, with respect to such values, a practical image noise level becomes greater or smaller than an image noise value expected under a predetermined scan condition, depending on differences with respect to each subject in the size of imaging sites or differences in morphological structure as well as tissue type. Therefore, there has been a problem that, if the practical image noise amount is greater than an allowable value, required diagnostic image quality cannot be obtained. In addition, there has been a problem that, in the case where the practical image noise level is smaller than an allowable value for diagnostic quality, the X-ray exposure dose becomes greater than is ideally necessary. Automatic tube current modulation (ATCM), that utilizes attenuation information from scout view(s) has been used for CT imaging of body that has a long scan range with variable attenuation, but it is not suited for CT of the heart that has short scan range with relatively uniform attenuation. In addition, since it dose not use information directly from reconstructed CT image that is a result of the use of various imaging conditions and methods described above, it does not accurately modulate radiation dose of post-contrast image.
Among the different types of CT studies, the reduction of radiation exposure is particularly important in coronary CT angiography. It uses relatively large radiation dose (equivalent to 300-650 chest x-ray examinations). It has been difficult to avoid X-ray exposure to the breasts, which are sensitive to radiation. Because of their anatomical position, it has been difficult to carry out precise measurement based on an element such as body weight, especially when there are individual differences in the size of the breasts and in the type of tissue (gland or fat) that is present, and there has been a need for precise modulation by the use of a standard deviation of CT number (also called Hounsfield number) directly obtained from reconstructed pre-contrast images that reflects scan parameters as well as size and tissue type of imaging part (heart and overlying breasts). The exponential relationship between body size, radiation dose and noise (standard deviation of CT number has long been established (Formula 1) and used for measurement of noise level on reconstructed image that contain homogenous fluid or tissue.